Structure analysis of triterpene saponins in Polygala tenuifolia by electrospray ionization ion trap multiple-stage mass spectrometry

Eighteen different triterpene saponins isolated from Polygala tenuifolia were investigated by electrospray ionization ion trap multiple-stage mass spectrometry (ESI-ITMSn) in positive and negative ion modes. MS1-MS3/MS4 spectra of the both modes were analyzed, and they all gave fragments in line and shared common fragmentation patterns. Key fragments from MSn spectra of both the modes and their proposed fragmentation pathways were constructed with examples illustrated for the formation of characteristic fragments in the saponins. Two special fragmentation patterns were proposed: (1) the formation of fragments by cleavage of CH2O from Delta(12)-14 alpha-CH2OH of the oleanene-type saponin aglycone in both positive and negative MSn (n >= 2) modes; (2) the occurrence of fragments by cleavage of CO2 and 3-glucose as the characteristic structure feature of 23-COOH at the oleanene-type saponin aglycones coupled with 3-Glc substitutes in the negative MSn (n >= 2) modes. Peak intensities in MSn spectra were also correlated with structural features and fragmentation preferences of the investigated saponins, which are discussed in detail. In general, fragments formed predominantly by cleavages of glycosidic bonds in the positive mode, while selective cleavages of acyl bonds preceded that of glycosidic bonds in negative MSn (n >= 2) mode, both of which could well be applied to the structural analysis of these saponins. Interpretation of MSn spectra presented here provided diagnostic key fragment ions important for the structural elucidation of saponins in P.tenuifolia. Copyright (c) 2007 John Wiley & Sons, Ltd.